<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>三角形</title>
</head>
<body>
    <div></div>
</body>
<style>
    div{
        height: 0;
        width: 0;
        border: 12px solid red;
        border-left: 12px solid transparent;
        border-right: 12px solid rgba(0,0,0,0);
        border-bottom: 12px solid transparent;
        margin: 200px auto;
    }
</style>
</html>